Fluid catalytic cracking (FCC) is one of the most important processes both in the United States and elsewhere for cracking petroleum feedstocks. In the FCC, a particulate catalyst is subjected to a continuous cyclic cracking reaction and catalyst regeneration procedure. In the reactor, the fluidized catalyst is contacted with a hydrocarbon feed usually at an average reactor temperature between about 800.degree. F. and 1100.degree. F. The reactions that occur in the reactor result in the deposition of a carbonaceous residue or coke on the catalyst particles. The cracked, or processed, hydrocarbon stream is separated from the coked catalyst and withdrawn from the cracking zone. The coked catalyst is stripped of volatiles in a steam stripping zone and passed to a catalyst regeneration zone. In the catalyst regeneration zone, the coked catalyst is contacted with a gas containing molecular oxygen to burn off a desired portion of the coke from the catalyst and simultaneously to heat the catalyst to a high temperature before the regenerated catalyst is recycled to the cracking zone.
In the typical FCC process, the particulate cracking catalyst is composed principally of silica and alumina. It may be an amorphous mixture of silica and alumina, but more likely, the catalyst will contain a crystalline aluminosilicate zeolite in an amorphous silica-alumina matrix. Conventional zeolitic cracking catalysts often include an X-type zeolite or a Y-type zeolite. The ability of these catalysts to perform their function is reduced by the presence of coke which is deposited on the particls during the cracking step. Although the coke may be removed during regeneration, excessive amounts will lead to deactivation of the catalyst resulting in lower activity and poor selectively to the desired products. Thus, most modern cracking catalysts are limited to cracking hydrocarbon charges having low coke forming tendency.
With the increasing interest in upgrading residual stocks and heavy crudes, it has become desirable to find ways for pretreating the feedstocks to lower the metals and Ramsbottom carbon levels in the feed before the cracking step. Metal contaminants such as iron, nickel and vanadium may act as poisons to the cracking catalysts and cause formation of excessive amounts of coke and gas. Ramsbottom carbon leads to the formation of coke on the catalyst particles and reduces catalyst activity. In one method of pretreatment, the feedstock is contacted with relatively inert fluidizable solid particles at a high temperature and for a brief residence time. In so doing, Ramsbottom carbon and metals content of the feedstock are lowered to the point where the pretreated feedstock may be fed to a conventional FCC unit. See U.S. Pat. No. 4,243,514.
In general, the coke forming tendency or coke precursor content of an oil can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolyzed. The industry accepts this value as a measure of the extent to which a given oil tends to form coke when employed as feedstock in a catalytic cracker. Two established tests are recognized, the Conradson carbon and the Ramsbottom carbon tests. The latter standard is described in ASTM Test No. D-524-76 and will be used in the following discussion.
In a common refinery situation, the feed to a fluid catalytic cracker will contain some material boiling in the range of desirable products. For example, the feed may contain material boiling in the range of diesel and jet fuel stocks. When contacted with active, regenerated catalyst in the FCC riser, this material undergoes a significant amount of dehydrogenation which reduces its value as diesel or jet fuel. Some of the material is also cracked to less desirable light gases.